git-merge-base(1) Manual Page

NAME

SYNOPSIS

DESCRIPTION

git merge-base finds best common ancestor(s) between two commits to use
in a three-way merge. One common ancestor is better than another common
ancestor if the latter is an ancestor of the former. A common ancestor
that does not have any better common ancestor is a best common
ancestor, i.e. a merge base. Note that there can be more than one
merge base for a pair of commits.

Unless --octopus is given, among the two commits to compute the merge
base from, one is specified by the first commit argument on the command
line; the other commit is a (possibly hypothetical) commit that is a merge
across all the remaining commits on the command line. As the most common
special case, specifying only two commits on the command line means
computing the merge base between the given two commits.

As a consequence, the merge base is not necessarily contained in each of the
commit arguments if more than two commits are specified. This is different
from git-show-branch(1) when used with the --merge-base option.

OPTIONS

-a

--all

Output all merge bases for the commits, instead of just one.

--octopus

Compute the best common ancestors of all supplied commits,
in preparation for an n-way merge. This mimics the behavior
of git show-branch --merge-base.

--independent

Instead of printing merge bases, print a minimal subset of
the supplied commits with the same ancestors. In other words,
among the commits given, list those which cannot be reached
from any other. This mimics the behavior of git show-branch
--independent.

DISCUSSION

Given two commits A and B, git merge-base A B will output a commit
which is reachable from both A and B through the parent relationship.

For example, with this topology:

o---o---o---B
/
---o---1---o---o---o---A

the merge base between A and B is 1.

Given three commits A, B and C, git merge-base A B C will compute the
merge base between A and a hypothetical commit M, which is a merge
between B and C. For example, with this topology:

o---o---o---o---C
/
/ o---o---o---B
/ /
---2---1---o---o---o---A

the result of git merge-base A B C is 1. This is because the
equivalent topology with a merge commit M between B and C is: